The introduction of hydraulically driven extension fractures from wells into hydrocarbon-bearing formations to enhance the rate of the recovery of hydrocarbons is a well-known and common practice. Hydraulic fracturing of the well involves the raising of fluid pressure in a section of the well bore by pumping through perforations in the well casing, or, in open holes, by isolating the formation to be pressurized by the use of inflatable packers or some other means. Once initiated, a fracture will propagate when the stresses acting perpendicular to the fracture tip are exceeded by the fluid pressure within the fracture at that location.
It is desirable that the hydraulic fracture remains within the hydrocarbon-bearing formation and does not extend vertically into adjacent overlying and/or underlying non-hydrocarbon bearing formations or strata. Maintaining the hydraulic fracture within the hydrocarbon-bearing formation or strata results in gaining the maximum enhancement in productivity and avoiding the formation of a connection from the well borehole to formations likely to yield water to the producing well thereby diluting or even displacing the hydrocarbons flowing into the well. When the fracture propagates, usually generally vertically, into such overlying or underlying non-producing or water bearing horizons, in the worst case, the well may become non-productive and a new well will have to be drilled. Even in less damaging circumstances, the well may be much less productive than the anticipated enhancement would call for. In situations where the overlying or underlying strata will not produce water, it is still undesirable to propagate the fracture into such strata because the expenditures for creating the fracture will have been largely wasted on non-productive formations.
It is also very desirable to know the permissible fluid injection pressures for fracturing in advance because this will aid in the design of the fracture treatment, including the estimation of the number of pumping units required.
Hydraulically driven extension fractures will propagate when the fluid pressure in the fracture exceeds the least principal compressive stress, S.sub.3, in the strata. Accordingly, when hydraulic fracturing is carried out, it is desired that the fluid pressure in the fracture be greater than the least principal compressive stress, S.sub.3, of the hydrocarbon-bearing strata, but less than the least principal compressive stress, S.sub.3, of both the adjacent overlying and underlying non-productive strata. Such conditions confine the hydraulically driven extension fractures to propagate only within the hydrocarbon-bearing strata.
For initiation of hydraulic fractures, the fluid pressure in the borehole must overcome the stress concentration produced by the presence of the hole in the rock strata and the tensile strength of the rock (see, e.g., M. K. Hubbert and D. G. Willis, 1957, Mechanics of Hydraulic Fracturing, AIME Trans., v. 210). Typically, the fluid pressure rises to a value exceeding S.sub.3 before the fracture initiates. Upon propagation out to some distance exceeding a few well-bore diameters, the well-bore fluid pressure required for continued propagation will fall to a lower value, slightly above S.sub.3. When, however, proppants are added to the fluid injected, as would typically be the case, high-velocity flow with attendant large pressure drop along the fracture is necessary to maintain the proppant in suspension. Thus, it is highly desirable that the injection pressure be as great as possible but still less than that required to propagate the fracture into the overlying or underlying strata.
Methods for measuring the state of stress in hydrocarbon-bearing formations which involve the hydraulic fracturing process itself have been widely reported in the literature. S.sub.3 can be readily determined by measuring the fluid pressure at which fracture propagation ceases. The most common measuring technique is as follows. First, initiate the fracture by pumping to increase the fluid pressure. Then shut off the fracturing pump. The fluid pressure drops sharply because of continuing flow into the fracture. Upon closure of the fracture, the fluid pressure ceases to fall rapidly and this, so-called, instantaneous shut-in pressure, ISIP, is taken to be the least principal compressive stress, S.sub.3.
Therefore, the least principal compressive stress S.sub.3. of the hydrocarbon-bearing strata or formation can be determined readily once the hydraulic fracturing operation is completed. The least principal compressive stress of the non-productive overlying and underlying strata are not measured in normal practice. It is significant to bear in mind that although the least principal compressive stress S.sub.3 of the hydrocarbon-bearing strata or formation can be measured once a hydraulic fracture operation is completed, S.sub.3 is not known in advance. It is a costly undertaking to hydraulically fracture each formation for direct measurement of S.sub.3 prior to hydraulically fracturing the hydrocarbon-bearing formation of the well.
A common commercial method used to predict stress state is known as FracHite which is proprietary to Schlumberger Technology Corporation. This method relies upon an equation which yields S.sub.3 incorporating the parameters of depth and density to yield the vertical stress, Sv, and Poisson's ratio, v, which is given by measurement of the compressional and shear wave velocities in the formations from wire-line sonic logging methods and a number of other parameters. For example, the predictive algorithm includes estimates of the vertical stress intensity factor or fracture toughness. The FracHite method requires the principal assumption that the horizontal stress within the formations is generated by confinement at some distant vertical boundary of the lateral Poisson's expansion caused by the superincumbent loading of the overlying sedimentary rock.
The present invention involves the discovery that this commercial method and its assumptions can be improved upon for providing S.sub.3 predictions in some of the most productive oil and gas provinces of the United States and elsewhere.